Israeli air strikes on Damascus and the conflicting reports on the use of chemical weapons (sarin gas) may complicate President Obama's decision on intervention in Syria. The US must consider the international laws of war before taking any action.

Damaged buildings wrecked by an Israeli airstrike are seen in Damascus, May 5. Israeli warplanes struck areas in and around the Syrian capital early Sunday, setting off a series of explosions as they targeted a shipment of highly accurate, Iranian-made guided missiles believed to be on their way to Lebanon's Hezbollah militant group, officials and activists said.

SANA (Syrian official news agency)/AP

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La Jolla, Calif. — Everyone seems to agree that the situation in Syria is unimaginably horrific and heartrending. But the consensus seems to break down when the subject of solutions is broached. Now, the reported use of chemical weapons (sarin gas) raises the stakes of the crisis – and outside intervention – considerably.

President Obama, who warned that the use of chemical weapons would be a “game changer,” is likely considering some kind of response beyond the nonlethal aid already given to Syria’s rebels. Alleged Israeli strikes on Damascus over the weekend may complicate matters. And many questions remain. One of the most important deals with whether US intervention in Syria would be “legal” under the UN Charter without Security Council backing.

And that legality matters. It can determine the costs of and allies involved in an intervention, set precedents for future military campaigns, and can increase or decrease the likelihood of future wars in general.

Even if the Security Council doesn’t sanction a Syrian intervention, any move by the United States to “put boots on the ground” in Syria could still be well supported by the international laws of war – and the demands of the UN Charter. And intervention to protect Syrian civilians may finally pressure Russia to finally give UN Security Council support for such a move.

However, consistent vetoes from Russia at the UN Security Council on further action in Syria make it unlikely the international body will back any military intervention there – at least for the time being. This is unfortunate, as the UN is the most prominent international organization and therefore shouldn’t be consigned to irrelevance as Syria is turned into a charnel house.

But action outside the framework of the UN – i.e., unilateral action – appears increasingly likely. Many will argue that the use of force in the absence of Security Council authorization (other than self-defense) is illegal; others will stress the primacy of human rights. It might be that the absence of Security Council authorization will render any operation illegal under international law, but that same law (including the UN Charter) obligates member states to act in the face of mass atrocities and large-scale human suffering.

If Mr. Obama and other international leaders pursue military intervention in Syria, they will ideally be guided by a concept known within the law of war as “jus ad bellum.” That is, the conditions under which a state is justified in resorting to war in the first place.

In order for the use of force to be justified, the following five criteria must be consulted: the seriousness of the harm; the primary purpose of the proposed action; the existence and viability of peaceful alternatives; the proportionality of the response; and, finally, the balance of consequences.

The seriousness of harm. In the case of Syria, the world is witnessing savagery and butchery the likes of which we haven’t seen since Rwanda. More than 70,000 people have been killed; civilians have been deliberately targeted; and the number of internally displaced persons now stands at more than 3 million, a situation the UN high commissioner for refugees calls the worst humanitarian disaster since the end of the cold war. This element, in other words, is not in doubt.

The primary purpose. The primary purpose of the intervention must be to halt the suffering. To be sure, ancillary considerations (for example deterring Iran, a strong ally of the Assad regime in Syria) can be present. But these considerations must not constitute the crux of the operation.

All publicly available comments from the White House indicate that any escalation of US force in Syria would be in the form of increased assistance to the rebels fighting Mr. Assad, the ultimate purpose of which would be to halt his ability to inflict more pain and suffering.

Peaceful alternatives. Obama must consider the existence of peaceful alternatives before committing to a more robust military response. Secretary of State John Kerry has said that the United States remains interested in facilitating a political transition whereby Assad would voluntarily go into exile. After more than two years of conflict, however, it appears certain that the regime will instead fight to the death.

A proportional response. Whatever course of action Obama decides to take must be proportional. That is, the assistance given must be tied to the ultimate goal of the operation (the cessation of suffering). Anything beyond what is militarily necessary to accomplish that goal would be unjustified.

Balance of consequences. Obama and his national security staff must consider the extent to which intervention might actually exacerbate the situation in Syria. In other words, could boots on the ground or missile strikes cause the regime in Damascus to double down by unleashing the full extent of its chemical weapons arsenal? Or alternatively, could the overt involvement of the US on the side of the rebels increase the possibility that Iran would intervene to protect its most important ally, thus triggering a regional conflict?

As we know from painful experience, plunging headlong into the affairs of another state based on incomplete intelligence is not a prudent course of action. Obama is rightfully reviewing the intelligence reports and consulting with allies before making any decisions. Americans should expect nothing less.

Having legal justification for an intervention is vital. The 1999 US-NATO intervention in the Kosovo crisis was deemed technically “illegal” under international law but legitimate according to the criteria above. The absence of full legality, however, weakened the case for intervention, even though most people agree that it was morally right to act. It’s known as the Kosovo exception, and many argue that it led to the 2003 invasion of Iraq.

Creating more loopholes and exceptions to the UN Charter through unilateral action undermines its legal framework and thus increases the likelihood that others will claim similar dispensations.

An “illegal” action will also doubtless be more expensive for the country engaging in it (see Iraq), as others will not be available to assist. Finally, many argue that a strict adherence to the rules makes war less likely, and therefore minimizes the chances that the horrors of war will be visited upon ordinary citizens.

It is possible, of course, that if the US and others move forward with a military intervention to protect the people of Syria, the action may push Russia to relent in its objections, thereby allowing the Security Council to be involved in whatever response is warranted.

But if Russia's cooperation is not forthcoming, Obama will be faced with a politically unpalatable predicament: increasing the involvement of the US military in Syria’s civil war without the blessing of the Security Council and in the face of public opposition or losing face by not following through on his own red line (the use of chemical weapons).

In any case, it seems likely that some kind of military response is in the works, so Obama’s reliance on the law of war criteria noted above will be especially important. How Obama responds will have long-lasting repercussions not only for Syria, Americans, and his legacy, but the precedents of international law as well.